Energy transfer amplification for intrabody devices

ABSTRACT

Apparatus for driving current in a power circuit of a medical device inserted into a body of a subject includes a power transmitter, which is adapted to generate, in a vicinity of the body, an electromagnetic field having a predetermined frequency capable of inductively driving the current in the power circuit. A passive energy transfer amplifier, having a resonant response at the frequency of the electromagnetic field is placed in proximity to the medical device so as to enhance the current driven in the power circuit by the electromagnetic field.

FIELD OF THE INVENTION

The present invention relates generally to position sensing systems, andspecifically to methods and devices for enhancing the transfer of energyto wireless position sensing systems inserted into the human body.

BACKGROUND OF THE INVENTION

Many surgical, diagnostic, therapeutic and prophylactic medicalprocedures require the placement of objects such as sensors, implants,tubes, catheters and treatment devices within the body. These procedurescover a large spectrum including, for example: a) placement of tubes tofacilitate the delivery of drugs, nutrition or other fluids into apatient's circulatory or digestive system, b) placement of tubes tofacilitate respiration of a patient, c) placement of tubes to facilitateremoval of gastrointestinal tract contents for analysis and/ortreatment, d) insertion of probes or surgical devices for diagnostic ortherapeutic treatments, and e) placement of orthopedic devices such asartificial hips and knees.

In some instances, placement of a device is for a limited time, such asduring surgery or catheterization. In other situations, such as withfeeding tubes or orthopedic implants, the devices are to be used forlong-term treatment. It is beneficial to provide real-time informationfor accurately determining the location and orientation of objects bothduring and after implantation within a patient's body, preferablywithout using X-ray imaging.

Noninvasive techniques for determining the location and orientation ofimplanted devices are often preferred, as such techniques are generallymore comfortable for the patient and easier to perform. Field sensors,such as Hall effect devices, coils or antennae, have been included inmedical devices to allow for noninvasive monitoring of the position ofthe devices.

PCT Patent Publication WO 96/05768 to Ben-Haim et al. and correspondingU.S. patent application Ser. No. 08/793,371, which are assigned to theassignee of the present patent application and which are incorporatedherein by reference, describe a locating system for determining thelocation and orientation of an invasive medical instrument, whereby anexternally-applied RF field induces a current in three coils locatedwithin the invasive medical instrument. Wires or some other form ofphysical leads are used to carry this induced signal from the catheterto a signal processor in the extrabody space. The processor analyzes thesignal so as to calculate the location and orientation of the invasivemedical instrument.

In many applications, a wireless passive emitter, or “tag,” is affixedto a device that is inserted into the body. Such a tag contains nointernal power source, but is rather actuated by an external energyfield, typically applied from outside the body. The tag then emitselectromagnetic energy, which is detected by antennas or other sensorsoutside the body. The detected signals are generally used simply toascertain the presence of the tag within a given region (such as theabdominal cavity), although some tags may also be used to determineposition coordinates. Some passive tags receive and re-emitelectromagnetic radiation, typically with a frequency and/or phaseshift.

For example, U.S. Pat. No. 6,026,818 to Blair et al., whose disclosureis incorporated herein by reference, describes a method and device forthe detection of unwanted objects in surgical sites, based on amedically-inert detection tag which is affixed to objects such asmedical sponges or other items used in body cavities during surgery. Thedetection tag contains a single signal emitter, such as a miniatureferrite rod and coil and capacitor element embedded therein.Alternatively, the tag includes a flexible thread composed of a singleloop wire and capacitor element. A detection device is utilized tolocate the tag by pulsed emission of a wide-band transmission signal.The tag resonates with a radiated signal, in response to the wide-bandtransmission, at its own single non-predetermined frequency, within thewide-band range. The return signals build up in intensity at a single(though not predefined) detectable frequency over ambient noise, so asto provide recognizable detection signals.

U.S. Pat. No. 5,057,095 to Fabian, whose disclosure is incorporatedherein by reference, describes apparatus for detecting a surgicalimplement in human or animal tissue, comprising a detector that isresponsive to the presence, within an interrogation zone, of a surgicalimplement to which a marker is secured. The marker is adapted to produceidentifying signal characteristics within a frequency band generated inthe interrogation zone. Variations in the phase and or direction of theinterrogating field and changes in the electromagnetic coupling betweenmarkers and receiver are intended to optimize coupling therebetween.

U.S. Pat. No. 6,076,007 to England et al., whose disclosure isincorporated herein by reference, describes a method for determining theposition and orientation of a surgical device within a human body. Inone application, a catheter or prosthesis is characterized in that itcarries, at a predetermined location, a tag formed of a highpermeability, low coercivity magnetic material. The position of the tag(and hence of the surgical device) is sensed by remotely detecting itsmagnetic response to an interrogating signal.

U.S. Pat. No. 5,325,873 to Hirschi et al., whose disclosure isincorporated herein by reference, describes a system to verify thelocation of a tube or other object inserted into the body. Itincorporates a resonant electrical circuit attached to the object whichresonates upon stimulation by a hand-held RF transmitter/receiverexternal to the body. The electromagnetic field generated due toresonance of the circuit is detected by the hand-held device, whichsubsequently turns on a series of LEDs to indicate to the user thedirection to the target. An additional visual display indicates when thetransmitter/receiver is directly above the object.

Passive sensors and transponders, fixed to implanted devices, can alsobe used for conveying other diagnostic information to receivers outsidethe body. For example, U.S. Pat. No. 6,053,873 to Govari et al., whosedisclosure is incorporated herein by reference, describes a stentadapted for measuring a fluid flow in the body of a subject. The stentcontains a coil, which receives energy from an electromagnetic fieldirradiating the body so as to power a transmitter for transmitting apressure-dependent signal to a receiver outside the body.

As another example, U.S. Pat. No. 6,206,835 to Spillman et al., whosedisclosure is incorporated herein by reference, describes an implantdevice that includes an integral, electrically-passive sensing circuit,communicating with an external interrogation circuit. The sensingcircuit includes an inductive element and has a frequency-dependentvariable impedance loading effect on the interrogation circuit, varyingin relation to the sensed parameter.

U.S. Pat. Nos. 5,391,199 and 5,443,489 to Ben-Haim, whose disclosuresare incorporated herein by reference, describe systems wherein thecoordinates of an intrabody probe are determined using one or more fieldsensors, such as a Hall effect device, coils, or other antennae carriedon the probe. Such systems are used for generating three-dimensionallocation information regarding a medical probe or catheter. Preferably,a sensor coil is placed in the catheter and generates signals inresponse to externally-applied electromagnetic fields. Theelectromagnetic fields are generated by three radiator coils, fixed toan external reference frame in known, mutually-spaced locations. Theamplitudes of the signals generated in response to each of the radiatorcoil fields are detected and used to compute the location of the sensorcoil. Each radiator coil is preferably driven by driver circuitry togenerate a field at a known frequency, distinct from that of otherradiator coils, so that the signals generated by the sensor coil may beseparated by frequency into components corresponding to the differentradiator coils.

U.S. Pat. No. 6,198,963 to Ben-Haim et al., whose disclosure isincorporated herein by reference, describes apparatus for confirmationof intrabody tube location. The initial location of the object isdetermined as a reference point, and subsequent measurements are made todetermine whether the object has remained in its initial position.Measurements are based upon one or more signals transmitted to and/orfrom a sensor fixed to the body of the object whose location is beingdetermined. The signal could be ultrasound waves, ultraviolet waves,radio frequency (RF) waves, or static or rotating electromagneticfields.

Other devices comprise multiple transducers which each perform specifictasks. For example, U.S. Pat. No. 6,239,724 to Doron et al., which isincorporated herein by reference, describes an implantable telemetrydevice that contains one transducer for converting a power signal fromoutside the body into electrical power for the device, a secondtransducer for receiving a position field signal form outside the body,and a third transducer for transmitting a signal to a site outside thebody.

U.S. Pat. No. 6,172,499 to Ashe, whose disclosure is incorporated hereinby reference, describes a device for measuring the location andorientation in the six degrees of freedom of a receiving antenna withrespect to a transmitting antenna utilizing multiple-frequency ACmagnetic signals. The transmitting component consists of two or moretransmitting antennae of known location and orientation relative to oneanother. The transmitting antennae are driven simultaneously by ACexcitation, with each antenna occupying one or more unique positions inthe frequency spectrum. The receiving antennae measure the transmittedAC magnetic field plus distortions caused by conductive metals. Acomputer then extracts the distortion component and removes it from thereceived signals, providing the correct position and orientation output.

U.S. Pat. No. 6,261,247 to Ishikawa et al., which is incorporated hereinby reference, describes an implantable position sensing systemcomprising a plurality of spherical semiconductors, which are capable ofdetermining their relative positions and communicating this informationamong each other and to an external processing unit. Radio frequencysignals are used for communication and to power the implanted sphericalsemiconductors.

Current intrabody radio frequency powered position sensors are limitedin the amount of power they can receive, due to their typically smallsize, as induced current varies with cross-sectional area of thereceiving coil. Additional power would be desirable, e.g., so thatadditional computations can be performed by the sensor and/or toincrease the strength of the signal transmitted by the sensor.

SUMMARY OF THE INVENTION

It is an object of some aspects of the present invention to provideimproved apparatus and methods for wirelessly transmitting power to anintrabody object.

It is a further object of some aspects of the present invention toprovide improved apparatus and methods for real-time determination ofthe location and orientation of intrabody objects.

It is yet a further object of some aspects of the present invention toprovide improved position measurement apparatus and methods based onradio frequency electromagnetic signals.

In preferred embodiments of the present invention, energy transfer froman external radio frequency (RF) power transmitter to a wirelessintrabody medical device is enhanced by a passive energy transferamplifier. The amplifier is positioned in proximity to the wirelessdevice so as to enhance inductive driving of a power coil in the deviceby the external driver. Typically, the energy transfer amplifiercomprises an amplification coil with a capacitance chosen so as todefine a resonant circuit with a desired resonant frequency. Theexternal RF power transmitter transmits a RF signal at the resonantfrequency, which drives a current in the amplification coil of theenergy transfer amplifier. The amplification coil preferably has acharacteristic diameter substantially larger than that of the powercoil. Therefore, the amplification coil re-radiates the RF signal withenhanced local field strength, and thus induces a greater current in thepower coil of the wireless medical device than would be provided by theexternal RF driver alone.

The term “passive,” as used in the context of the present patentapplication and in the claims, refers to electrical units that do notcomprise an active power source, such as a battery, and are not wired tosuch a power source. In the embodiments described herein, such passiveunits have the advantages of reduced size and essentially unlimiteduseful life. Thus, in some embodiments of the present invention, passiveenergy transfer amplifiers are implanted in a patient's body inproximity to other wireless devices. In other embodiments, passiveenergy transfer amplifiers may be placed on or adjacent to the patient'sbody, in order to boost the RF power transferred to a wireless deviceinside the body. These embodiments enable the system operator (such as asurgeon) to place the bulky RF power transmitter, along with itsassociated wiring, relatively far from the site of the wireless devicewithin the body, so that the transmitter is less likely to interferewith a medical or surgical procedure using the wireless device.

In some preferred embodiments of the present invention, the medicaldevice comprises one or more wireless position sensors, which are usedto determine the location and/or orientation of the intrabody medicaldevice. In a preferred embodiment, the medical device comprises a hipjoint implant, including a femur head and an acetabulum. A signalprocessor, typically external to the patient, is adapted to determine adistance between the femur head and the acetabulum responsive to theoutput signals from the position sensors. Preferably, the energytransfer amplifier is implanted in the acetabulum, in close proximity tothe position sensors, so as to improve the wireless transfer of energyto the position sensors. Since the energy transfer amplifier iscompletely enclosed by the implanted acetabulum, no additional surgicalprocedure is necessary to implant the apparatus. Moreover, since theenergy transfer amplifier is isolated from bodily tissues,biocompatibility issues are generally minimal. Alternatively, the energytransfer amplifier is implanted in body tissue adjacent to the hip jointimplant. Further alternatively, the energy transfer amplifier is affixedexternally to the patient adjacent to the hip joint implant. Preferably,the position sensors are used to properly orient the femur head and theacetabulum during implant surgery and to confirm that the properorientation is maintained during the post-surgical period.

In another preferred embodiment, the medical device comprises anartificial knee. The signal processor, as described hereinabove, isadapted to determine the relative positions of the articulating portionsof the artificial knee responsive to the output signals from theposition sensors. Preferably, the energy transfer amplifier is implantedin or placed near the artificial knee, in close proximity to theposition sensors. The position information thus generated is used toproperly place the artificial knee during surgery and to monitor theknee post-operatively.

In some further preferred embodiments of the present invention, theenergy transfer amplifier is used to improve the wireless transfer ofpower to implantable physiologic monitors. In a preferred embodiment, animplantable cardiac monitor is used to track the functioning of apatient's heart and report to a physician via continuous or periodicdownloads of data to an external analysis device, such as a computer.The energy transfer amplifier is used to improve the wireless transferof energy to the implanted device such that more power is available totransmit the data to the analysis device. The improved energy transfercan be used in the immediate transfer of data and/or to charge energystorage devices such as batteries or capacitors. Alternative physiologicmonitors comprise: blood flow monitors, blood oxygenation monitors, andblood insulin level monitors.

In additional preferred embodiments of the present invention, the energytransfer amplifier is used to improve the wireless transfer of power toposition sensors incorporated into invasive probes, such as cathetersthat are used for mapping of electrical activity in a heart. In apreferred embodiment of the present invention, the energy transferamplifier is positioned on skin of a subject in a vicinity of the heartto improve the wireless transfer of energy to one or more positionsensors incorporated into a catheter that has been inserted into theheart. Alternatively, the energy transfer amplifier is placed within thesubject's body, for example by being fixed to the catheter, or by beingtemporarily placed within a chamber of the subject's heart.

In still further preferred embodiments of the present invention, theenergy transfer amplifier is used to improve the wireless transfer ofpower to implantable electrical stimulation devices. In a preferredembodiment of the present invention, the energy transfer amplifier isimplanted adjacent to a cardiac pacemaker to improve the wirelesstransfer of energy to the pacemaker, so as to charge the power source ofthe pacemaker. Additional embodiments of the present invention compriseusing the energy transfer amplifier to improve the wireless transfer ofenergy to implantable devices for stimulating the brain or the pancreas.

There is therefore provided, in accordance with an embodiment of thepresent invention, apparatus for driving current in a power circuit of amedical device inserted into a body of a subject, the apparatusincluding:

a power transmitter, which is adapted to generate, in a vicinity of thebody, an electromagnetic field having a predetermined frequency capableof inductively driving the current in the power circuit; and

a passive energy transfer amplifier, having a resonant response at thefrequency of the electromagnetic field and adapted to be placed inproximity to the medical device so as to enhance the current driven inthe power circuit by the electromagnetic field.

Typically, the passive energy transfer amplifier includes a coil and acapacitance, which are coupled so as to define a resonant circuit havingthe resonant response at the frequency of the electromagnetic field.

There is also provided, in accordance with an embodiment of the presentinvention, apparatus for use in an invasive medical procedure,including:

a wireless medical device, which is adapted to be inserted into a bodyof a subject, the device including a power circuit, which is adapted tobe driven inductively by a radio-frequency (RF) electromagnetic field soas to provide operating energy to the device;

a power transmitter, which is adapted to generate the RF electromagneticfield in a vicinity of the body; and

a passive energy transfer amplifier, which is adapted to be placed inproximity to the medical device so as to enhance inductive driving ofthe power circuit of the wireless medical device by the RFelectromagnetic field.

In some embodiments, the passive energy transfer amplifier is adapted tobe implanted in the body in proximity to the medical device. In otherembodiments, the passive energy transfer amplifier is adapted to befixed externally to the body in proximity to an area of the body intowhich the medical device is inserted.

In some embodiments, the medical device includes a sensor, which isadapted to sense a parameter within the body, and a signal transmitter,which is coupled to transmit a signal indicative of the parameter to areceiver outside the body. In one embodiment, the power circuit of thewireless medical device includes a coil antenna for receiving theelectromagnetic field, and the signal transmitter is coupled to transmitthe signal via the coil antenna. Typically, the sensor includes aposition sensor, and the transmitted signal is indicative of positioncoordinates of the medical device within the body. In one embodiment,the position sensor includes a sensor coil, and the apparatus furtherincludes one or more field generators, which are adapted to generateenergy fields in a vicinity of the medical device, which cause currentsto flow in the sensor coil responsively to the position coordinates ofthe medical device.

In another embodiment, the parameter that is sensed by the sensorincludes a physiological parameter. Typically, the physiologicalparameter includes an electrical parameter or, alternatively oradditionally, at least one of a temperature, a pressure, a chemicalparameter and a flow parameter.

In a further embodiment, the medical device is adapted to apply at leasta portion of the operating energy to tissue in the body. Typically, themedical device includes an electrode, which is adapted to applyelectrical energy to the tissue.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for use in an invasive medical procedure,including:

a wireless medical device, which is adapted to be inserted into a bodyof a subject, the device including a power circuit, which is adapted tobe driven inductively by a radio-frequency (RF) electromagnetic fieldgenerated by a power transmitter outside the body, so as to provideoperating energy to the device; and

a passive energy transfer amplifier, which is adapted to be placed inproximity to the medical device so as to enhance inductive driving ofthe power circuit of the wireless medical device by the RFelectromagnetic field.

There is further provided, in accordance with an embodiment of thepresent invention, an orthopedic implant, including:

a prosthetic joint including first and second joint elements, which areadapted to be implanted in a body of a subject;

first and second wireless position sensors, which are respectively fixedto the first and second joint elements so as to transmit positionsignals indicative of an alignment of the first and second jointelements, each of the position sensors including a power circuit, whichis adapted to be driven inductively by a radio-frequency (RF)electromagnetic field so as to provide operating energy to the sensors;

a power transmitter, which is adapted to generate the RF electromagneticfield in a vicinity of the body; and

a passive energy transfer amplifier, which is fixed to at least one ofthe first and second joint elements so as to enhance inductive drivingof the power circuit of the wireless position sensors by the RFelectromagnetic field.

In one embodiment, the prosthetic joint includes a hip joint, and thefirst and second joint elements include a femur head element and anacetabulum element, and the passive energy transfer amplifier is fixedto the acetabulum element. In another embodiment, the prosthetic jointincludes a knee joint.

There is moreover provided, in accordance with an embodiment of thepresent invention, invasive medical apparatus, including:

a catheter, having a distal end, which is adapted to be inserted into aheart of a subject, the catheter including a wireless position sensor,fixed adjacent to the distal end of the catheter so as to transmitposition signals indicative of a position of the catheter within theheart, the position sensor including a power circuit, which is adaptedto be driven inductively by a radio-frequency (RF) electromagnetic fieldso as to provide operating energy to the position sensor;

a power transmitter, which is adapted to generate the RF electromagneticfield in a vicinity of the body; and

a passive energy transfer amplifier, which is adapted to be placed in avicinity of the heart so as to enhance inductive driving of the powercircuit of the wireless position sensors by the RF electromagneticfield.

Typically, the passive energy transfer amplifier is adapted to be placedon a chest of the subject adjacent to the heart.

In a disclosed embodiment, the wireless position sensor includes asensor coil, and the apparatus further includes one or more fieldgenerators, which are adapted to generate energy fields in a vicinity ofthe heart, wherein the energy fields cause currents to flow in thesensor coil responsively to the position coordinates of the medicaldevice.

Additionally or alternatively, the catheter further includes one or moreelectrodes for sensing electrical activity within the heart.

There is furthermore provided, in accordance with an embodiment of thepresent invention, a method for driving current in a power circuit of amedical device inserted into a body of a subject, the method including:

generating, in a vicinity of the body, an electromagnetic field having apredetermined frequency capable of inductively driving the current inthe power circuit; and

placing a passive energy transfer amplifier, having a resonant responseat the frequency of the electromagnetic field, in proximity to themedical device so as to enhance the current driven in the power circuitby the electromagnetic field.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing details of a an energytransfer amplifier and a wireless location sensor, in accordance with apreferred embodiment of the present invention;

FIG. 2 is a schematic, pictorial illustration showing the use of theenergy transfer amplifier and wireless location sensors of FIG. 1 in ajoint implant, in accordance with a preferred embodiment of the presentinvention; and

FIG. 3 is a schematic, pictorial illustration of a mapping system, formapping of electrical activity in a heart, in accordance with apreferred embodiment of the present invention; and

FIG. 4 is a schematic, pictorial illustration showing a distal portionof a catheter for use in the mapping system of FIG. 3, in accordancewith a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 (not to scale) is a schematic illustration of the interaction ofan energy transfer amplifier 40 with a sensor 20, in accordance with apreferred embodiment of the present invention. Preferably, energytransfer amplifier 40 comprises at least one wire loop 30 and an inlinecapacitance 32, in series therewith. Capacitance 32 may comprise adiscrete capacitor or, alternatively or additionally, an inherentcapacitance provided by the structure of loop 30 and other elements ofamplifier 40. Energy transfer amplifier 40 is used to enhance energytransfer to a sensor 20, which comprises a power coil 22. A powertransmitting unit 52 produces a radio frequency (RF) electromagneticfield 34 that drives a current in both power coil 22 and energy transferamplifier 40. Energy transfer amplifier 40, in turn, inductively drivespower coil 22. Wire loop 30 preferably has a characteristic diametersubstantially larger than that of power coil 22.

Preferably, inline capacitance 32 is chosen so that loop 30 andcapacitance 32 form a resonant circuit, having a resonance at thefrequency of electromagnetic field 34. This resonance increases theinductive driving of power coil 22, since it tends to generate astronger field in the vicinity of loop 30 (and particularly along theaxis of the loop) than would be generated by power transmitting unit 52on its own. For example, electromagnetic field 34 may have a frequencyof 13.6 MHz and capacitor 32 may be approximately 50 pF, so as to induceresonance in loop 30, and thereby drive loop 30 to also radiate at 13.6MHz. Other combinations of electromagnetic field frequency and capacitorstrength, chosen so as to cause resonance between transfer amplifier 40and the electromagnetic field, are used in other preferred embodiments.The large size of coil 30 has been shown to increase the current inpower coil 22 by at least 20 times, in experiments carried out by theinventor. Thus, the power available to drive sensor 20 is significantlyincreased.

In a preferred embodiment, the increased power available to sensor 20 isused to power circuitry such as a controller 24, which includes avoltage-to-frequency converter 28 to aid in transmitting measurementsfrom a sensing coil 26 out of the sensor via the power induced in powercoil 22. The extra power is also used to increase the signal strength ofthe outgoing sensor signal. For some applications, the increased poweravailable provided by use of amplifier 40 permits the use of alower-power electromagnetic field 34, which in turn generally reducesany interference in sensing coil 26 caused by magnetically-responsivearticles in the vicinity of sensor 20.

In a preferred embodiment of the present invention, at least one sensor20 is attached to or incorporated into a medical device for insertioninto or implantation in a subject. For some applications, sensor 20comprises a wireless position sensor. For other applications, sensor 20comprises another type of sensor or active element, such as a sensingelectrode, a temperature sensor, a chemical sensor, optical sensor oracoustic sensor. Alternatively or additionally, the medical device maycomprise an active element, such as a pacing electrode, an ablationelement, or an acoustic radiator, powered by coil 22.

FIG. 2 is a schematic, pictorial illustration showing a position sensingsystem 18 for use in an orthopedic procedure, in accordance with apreferred embodiment of the present invention. In this preferredembodiment, position sensing during surgery is facilitated in a patient64 who undergoes a hip replacement, in which an artificial femur 60 andan artificial acetabulum 62 are implanted.

It is considered beneficial during surgery to properly align the head offemur 60 in acetabulum 62 and to maintain proper alignment over time, inorder to increase the working lifespan of the implant. To facilitateproper alignment of femur 60 and acetabulum 62, a plurality of wirelessposition sensors 20 are preferably incorporated within femur 60 andacetabulum 62. Preferably, wireless position sensors 20 are located soas to aid in the alignment and orientation between femur 60 andacetabulum 62 during surgery, and/or to facilitate post-surgicalconfirmation of proper placement.

Typically, but not necessarily, techniques described in U.S. patentapplication Ser. No. 10/029,473 to Govari, filed Dec. 21, 2001,entitled, “Wireless position sensor,” which is assigned to the assigneeof the present patent application and incorporated herein by reference,are adapted for use with the techniques of these embodiments of thepresent invention. In particular, as shown in FIG. 1, each positionsensor 20 may comprise a wireless location transponder, comprising apower coil 22, a sensing coil 26, and a controller 24, as described inthe '473 application. As shown in FIG. 2, at least oneexternally-located power transmitting unit 52 sends a radio frequency(RF) signal, preferably having a frequency in the megahertz range, todrive power coil 22 in the transponder and thereby power the chip.Additionally, a set of electromagnetic field generators 50 in fixedlocations outside the body produce electromagnetic fields at different,respective frequencies, typically in the kilohertz range. These fieldscause currents to flow in sensing coil 26, which depend on the spatialposition and orientation of the sensing coil relative to the fieldgenerators. The processing chip converts these currents intohigh-frequency signals, which are transmitted by the power coil to anexternally-located signal processing unit 54. This unit processes thesignal in order to determine position and orientation coordinates of theobject for display and recording. Position sensor 20 is preferably ableto generate six position and orientation dimensions, using techniquesdescribed in the above-cited PCT Patent Publication to Ben-Haim et al.,or other techniques known in the art. Although position sensor 20 isdescribed hereinabove as comprising coils, position sensor 20 can alsocomprise field sensors other than coils, such as Hall effect devices orother antennae.

Preferably, wireless position sensors 20 are powered by an external 13.6MHz electromagnetic field. The power transmitted to sensors 20 generallydecays according to the cube of the distance between the source and thesensors, so there is typically a reduction in the power transmitted tothe sensors responsive to the distance. Additionally, because thecurrent induced in a coil varies linearly with the cross-sectional areaof the coil, and, since position sensors 20 are relatively small(typically on the order of a few millimeters), the induced current inthe power coils of the position sensors is significantly smaller than itwould be if the power coils were larger, making it correspondingly moredifficult to receive power at the sensors and/or to transmit signalsfrom the sensor.

To increase the power transmitted to sensors 20, an energy transferamplifier 40 is implanted adjacent to the sensors, preferably inacetabulum 62, such that it is in close proximity to the positionsensors, so as to improve the wireless transfer of energy to theposition sensors. Furthermore, in this case, since energy transferamplifier 40 is completely enclosed by implanted acetabulum 62, noadditional surgical procedure is necessary to implant the apparatus.Moreover, since the energy transfer amplifier is isolated from bodilytissues, biocompatibility issues are generally minimal. Energy transferamplifier 40 is preferably substantially larger than sensors 20. Whilesensors 20 are typically a few millimeters in size, coils in energytransfer amplifier 40 are typically about 5 to 10 centimeters indiameter. Since induced current is generally proportional to thecross-sectional area of a coil, the induced current in the energytransfer amplifier is typically much larger than the induced current ina coil contained in a sensor, for a given electromagnetic field.

In a preferred embodiment of the present invention, position sensor 20is attached to or incorporated in an artificial knee. Techniquesgenerally similar to those described hereinabove with reference to FIG.2 are preferably used. A signal processor is adapted to determine therelative positions of the two articulating portions of the artificialknee, responsive to the output signals from the position sensors.Preferably, the energy transfer amplifier is implanted in or placed nearthe artificial knee, in close proximity to the position sensors. Theposition information obtained in this manner is used to properly placethe artificial knee during surgery and to monitor the kneepost-operatively.

In another preferred embodiments of the present invention, the energytransfer amplifier is used to improve the wireless transfer of power toimplantable physiologic monitors. In a preferred embodiment, animplantable cardiac monitor is used to track the functioning of apatient's heart and report to a physician via continuous or periodicdownloads of data to an external analysis device, such as a computer.The energy transfer amplifier is used to improve the wireless transferof energy to the implanted device such that more power is available totransmit the data to the analysis device. The improved energy transfercan be used in the immediate transfer of data and/or to charge energystorage devices such as batteries or capacitors. Alternative physiologicmonitors include, but are not limited to, blood flow monitors, bloodoxygenation monitors, and blood insulin level monitors.

Reference is now made to FIGS. 3 and 4, which are schematic, pictorialillustration of a mapping system 120, for mapping of electrical activityin a heart 124 of a subject 126, and of a catheter 130 used in system120, in accordance with a preferred embodiment of the present invention.Catheter 130 is inserted by a user 122 through a vein or artery of thesubject into a chamber of the heart. The catheter has an array of shaftelectrodes 146 on its outer surface, most preferably in a gridarrangement as shown in FIG. 4. Alternatively, the electrodes maycomprise ring electrodes, or substantially any other suitable type ofsurface electrodes, as are known in the art. Additionally, the catheteroptionally has one or more tip electrodes 148, typically at or near adistal tip 144 of the catheter.

For some applications, at least one of the electrodes also functions asan ablating electrode, for ablating cardiac tissue of the subject.Alternatively, at least one separate electrode is provided for thispurpose (not shown).

Catheter 130 also comprises position sensors 140 and 142, preferably oneof them near distal tip 144 and the other near a proximal end of thearray of electrodes. The sensors preferably comprise wirelesstransponders, similar to sensors 20, as described hereinabove withreference to FIG. 1. The position sensors are fixed to or withincatheter 130 by any suitable method, for example, using polyurethaneglue or the like.

To use sensors 140 and 142, the patient is placed in an electromagneticfield generated, for example, by situating under the patient a padcontaining field generator coils 128 for generating magnetic fields.These fields cause currents to flow in sensing coils 26, as describedabove. A reference electromagnetic sensor (not shown) is preferablyfixed relative to the patient, e.g., taped to the patient's back, andcatheter 130 containing sensors 140 and 142 is advanced into thepatient's heart. The sensors are preferably powered by a powertransmitting unit 152, which produces an electromagnetic field thatdrives a current in power coil 22 (FIG. 1) of sensors 140 and 142 (FIG.4). Sensors 140 and 142 preferably transmit position signals using coil22, which signals are received by field generator coils 128. Thesesignals are amplified and transmitted to a computer housed in a console34 (FIG. 3), in a form understandable to the computer. The computeranalyzes the signals so as to determine and visually display the preciselocation of sensors 140 and 142 in the catheter relative to thereference sensor. The sensors can also detect displacement of thatcatheter that is caused by contraction of the heart muscle.

At least one passive energy transfer amplifier 40 is placed on or nearsubject 126, such as on skin of the subject near the heart. For example,amplifier 40 may be fixed to or within an adhesive patch, which is stuckonto the subject's chest in the proper location. The electromagneticfield produced by power transmitting unit 152 to power sensors 140 and142 also drives a current in amplification coil 30 of the energytransfer amplifier (FIG. 1), which, in turn, inductively drives powercoil 22 in sensors 140 and 142. Wire loop 30 preferably has acharacteristic diameter substantially larger than that of power coil 22.Thus, energy transfer amplifier 40 enhances energy transfer to thesensors from power transmitting unit 152.

Some of the features of catheter 130 and system 120 are implemented inthe NOGA-STAR catheter marketed by Cordis Webster, Inc., and in theabove-mentioned Biosense-NOGA system, also marketed by Cordis Webster,Inc. Further aspects of the design of catheter 130 and of system 120generally are described in U.S. patent application Ser. No. 09/506,766,which is assigned to the assignee of the present application and isincorporated herein by reference.

As noted above, sensors 140 and 142 each comprise one or more sensingcoils 26, which act as AC magnetic field receivers, to sense the ACmagnetic fields generated by field generator coils 128 (which are alsoreferred to as magnetic field transmitters or radiators). Coils 128generate AC magnetic fields to define a fixed frame of reference.Preferred implementations of sensing coils 26 and of position sensorsbased on such coils are further described in the above-mentioned U.S.Pat. No. 5,391,199 and PCT publication WO 96/96/05768. The position andorientation coordinates of the distal portion of catheter 130 areascertained by determining the position and orientation of sensors 140and 142 (through identifying the position and orientation coordinatesthereof) relative to coils 128. Sensors 140 and 142 may each comprise asingle sensing coil 26, as shown in FIG. 1, or may alternativelycomprise two, three or more sensor coils, wound on either air cores or acore of material.

In a preferred embodiment of the invention (not shown in the figures),the sensor coils have mutually orthogonal axes, one of which isconveniently aligned with the long longitudinal axis of catheter 130.Unlike prior art position sensors (used for other applications), whichcontain three coils that are concentrically located, or at least whoseaxes intercept, the sensor coils in the present embodiment are closelyspaced along the longitudinal axis of catheter 130 to reduce thediameter of sensors 140 and 142 and thus make the sensors suitable forincorporation into catheter 130 (which may also contain a lumen as aworking channel within the catheter).

It should be understood that placement of field generator coils 128, aswell as their size and shape, will vary according to the application ofthe invention. Preferably, in system 120, coils 128 comprise woundannular coils from about 2 to 20 cm in outer diameter (O.D.) and fromabout 0.5 to 2 cm thick, in a coplanar, triangular arrangement whereinthe centers of the coils are from about 2 to 30 cm apart. Bar-shapedtransmitters or even triangular or square-shaped coils could also beuseful for such medical applications. Moreover, in instances wheresubject 126 is prone, as shown in FIG. 3, field generators 128 arepreferably positioned in or below the surface upon which the subject isresting (such as an operating table), substantially directly below theportion of the subject's body 90 where the procedure is being performed.

Field generator coils 128 are driven by a radiator driver 132. Theposition signals transmitted by sensors 140 and 142 in response to thefields of coils 128 are received by coils 128 or by another receivingantenna. These signals are amplified and processed, together with arepresentation of the signals used to drive transmitters coils 128,preferably in the manner described below, by a console 134. The consoleprovides a display or other indication of the position and orientationof the distal end of catheter 130 on a monitor 136.

Field generator coils 128 may be arranged in any convenient position andorientation, so long as they are fixed in respect to some referenceframe, and so long as the coils are non-overlapping (i.e., there are notwo coils 128 with the same position and orientation). When driven byradiator driver 132, coils 128 generate a multiplicity ofdistinguishable AC magnetic fields that form the magnetic field sensedby sensing coils 126 in the sensors 140 and 142. The magnetic fields aredistinguishable with regard to the frequency, phase, or both frequencyand phase of the signals in the respective magnetic fields. Timemultiplexing is also possible.

In a preferred embodiment of the invention, sensors 140 and 142 insystem 120 are used to determine six position and orientationcoordinates (X, Y, Z directions and pitch, yaw and roll orientations) ofthe distal end and distal tip of catheter 130. For this purpose, atleast two sensing coils are typically required in the location sensors.Preferably, three sensing coils are used, as described above, to improvethe accuracy and reliability of the position measurement. Alternatively,if only a single sensing coil 26 is used in each sensor, the system 130may be able to determine only five position and orientation coordinates(X, Y, Z directions and pitch and yaw orientations). Specific featuresand functions of a position tracking system with a single sensing coil(also referred to as a single axis system) are described incommonly-assigned U.S. Pat. No. 6,484,118, which is incorporated hereinby reference.

In one embodiment of the invention, each sensing coil 26 has an innerdiameter of 0.5 mm and comprises 800 turns of 16 μm diameter to give anoverall coil diameter of 1-1.2 mm. The effective capture area of thesensing coil in this case is preferably about 400 mm². It will beunderstood that these dimensions may vary over a considerable range andare only representative of a typical range in certain embodiments. Inparticular, the size of the sensing coils can be as small as 0.3 mm(with some loss of sensitivity) and as large as 2 or more mm. In otherembodiments, the wire size of the sensing coils can range from 10 to 31μm, and the number of turns between 300 and 2600, depending on themaximum allowable size and the wire diameter. The effective capture areamay advantageously be made as large as feasible, consistent with theoverall size requirements.

While the preferred sensing coil shape is cylindrical, other shapes canalso be used. For example a barrel shaped coil can have more turns thana cylindrical shaped coil for the same diameter of catheter. Also,square or other shaped coils may be useful depending on the geometry ofcatheter 130. Location sensors 140 and 142 may be used to determine bothwhen the catheter is in contact with the tissue of heart 124 and alsowhen the heart is not in motion. During diastole, the heart isrelatively motionless for a short period of time (at most, a few hundredmilliseconds). In addition to using location sensors 140 and 142, thelocation of catheter 130 may be determined using outside sensing orimaging means.

As noted above, catheter 130 is coupled to console 134, which enablesthe user to observe and regulate the functions of the catheter. Console134 includes a processor, preferably a computer with appropriate signalprocessing circuits (which are typically contained inside a housing ofthe computer). The processor is coupled to drive monitor 136. The signalprocessing circuits typically receive, amplify, filter and digitizesignals from catheter 130, including signals generated by positionsensors 140 and 142 and electrodes 146 that are amplified by energytransfer amplifier 40. The digitized signals are received and used bythe console to compute the position and orientation of the catheter andto analyze the electrical signals from the electrodes. The informationderived from this analysis is preferably used to generate a map 138 ofthe heart's electrical activity.

Typically, system 120 includes other elements, which are not shown inthe figures for the sake of simplicity. Some of these elements aredescribed in U.S. Pat. No. 6,226,542 to Reisfeld, which is assigned tothe assignee of the present patent application and is incorporatedherein by reference. For example, system 120 may include an ECG monitor,coupled to receive signals from one or more body surface electrodes, soas to provide an ECG synchronization signal to console 134. As mentionedabove, the system typically also includes a reference position sensor,either on an externally-applied reference patch attached to the exteriorof the patient's body, or on an internally-placed catheter, which isinserted into heart 124 and maintained in a fixed position relative tothe heart. By comparing the position of catheter 130 to that of thereference catheter, the coordinates of catheter 130 are accuratelydetermined relative to the heart, irrespective of heart motion.Alternatively, any other suitable method may be used to compensate forheart motion.

As mentioned above, the information generated by catheter 130,facilitated by the operation of energy transfer amplifier 40, is used togenerate a map of electrical activity over an endocardial surface of thechamber. For some applications, methods described in U.S. Pat. Nos.6,226,542 and 6,301,496 to Reisfeld, which are assigned to the assigneeof the present patent application and are incorporated herein byreference, are used to generate the map. As indicated in these patents,location and electrical activity is preferably initially measured onabout 10 to about 20 points on the interior surface of the heart. Thesedata points are then generally sufficient to generate a preliminaryreconstruction or map of the cardiac surface to a satisfactory quality.The preliminary map is often combined with data taken at additionalpoints in order to generate a more comprehensive map of the heart'selectrical activity. In clinical settings, it is not uncommon toaccumulate data at 100 or more sites to generate a detailed,comprehensive map of heart chamber electrical activity. The generateddetailed map may then serve as the basis for deciding on a therapeuticcourse of action, for example, tissue ablation, which alters thepropagation of the heart's electrical activity and restores normal heartrhythm.

Alternatively or additionally, methods described in U.S. Pat. Nos.5,391,199 and 6,285,898 to Ben-Haim, U.S. Pat. Nos. 6,368,285 and6,385,476 to Osadchy et al., and/or in U.S. Pat. No. 6,400,981, toGovari. All these patents are assigned to the assignee of the presentpatent application and are incorporated herein by reference, are used.Further alternatively or additionally, other methods known in the artfor mapping the heart are used.

In a preferred embodiment of the present invention, the energy transferamplifier is used to improve the wireless transfer of power toimplantable electrical stimulation devices. In a preferred embodiment,the energy transfer amplifier is implanted adjacent to a cardiacpacemaker to improve the wireless transfer of energy to the pacemaker,so as to charge the power source of the pacemaker. Additionalembodiments of the present invention comprise using the energy transferamplifier to improve the wireless transfer of energy to implantabledevices for stimulating the brain or the pancreas.

It will thus be appreciated by persons skilled in the art that thepresent invention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and subcombinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

1. Apparatus for use in an invasive medical procedure, comprising: awireless position sensor, which is adapted to be inserted into a body ofa subject, the position sensor comprising a power circuit, which isadapted to be driven inductively by a radio-frequency (RF)electromagnetic field so as to provide operating energy to the positionsensor; a power transmitter, which is adapted to generate the RFelectromagnetic field in a vicinity of the body; and a passive energytransfer amplifier, which is adapted to be placed in proximity to theposition sensor so as to enhance inductive driving of the power circuitof the wireless position sensor by the RF electromagnetic field, theposition sensor transmitting signals for determining six position andorientation coordinates of the position sensor; and a signal processingunit for receiving signals from the position sensor and determining sixposition and orientation coordinates of the position sensor.
 2. Theapparatus according to claim 1, wherein the power transmitter is adaptedto generate the RF electromagnetic field at a predetermined frequency,and wherein the passive energy transfer amplifier has a resonantresponse at the predetermined frequency.
 3. The apparatus according toclaim 2, wherein the passive energy transfer amplifier comprises a coiland a capacitance, which are coupled so as to define a resonant circuithaving the resonant response at the predetermined frequency.
 4. Theapparatus according to claim 1, wherein the passive energy transferamplifier is adapted to be implanted in the body in proximity to thewireless position sensor.
 5. The apparatus according to claim 4, whereinthe wireless position sensor is for use in association with anorthopedic implant, and wherein the passive energy transfer amplifier isincorporated in the orthopedic implant.
 6. The apparatus according toclaim 5, wherein the implant is a hip joint implant, including a femurhead element and an acetabulum element, and wherein the passive energytransfer amplifier comprises a coil, which is integrated in theacetabulum element.
 7. The apparatus according to claim 1, wherein thepassive energy transfer amplifier is adapted to be fixed externally tothe body in proximity to an area of the body into which the wirelessposition sensor is inserted.
 8. The apparatus according to claim 7,wherein the wireless position sensor is fixed to an invasive probe forinsertion into a head of the subject, and wherein the passive energytransfer amplifier is adapted to be fixed to a chest of the subject. 9.The apparatus according to claim 8, wherein the wireless position sensoris adapted to provide an indication of location of the probe within theheart.
 10. The apparatus according to claim 1, wherein the power circuitof the wireless position sensor comprises a coil antenna for receivingthe electromagnetic field, and wherein the signal transmitter is coupledto transmit the signal via the coil antenna.
 11. The apparatus accordingto claim 1, wherein the position sensor comprises a sensor coil, andwherein the apparatus further comprises one or more field generators,which are adapted to generate energy fields in a vicinity of the medicaldevice, which cause currents to flow in the sensor coil responsively tothe position and orientation coordinates of the wireless positionsensor.
 12. Apparatus for use in an invasive medical procedure,comprising; a wireless position sensor, which is adapted to be insertedinto a body of a subject, the device comprising a power circuit, whichis adapted to be driven inductively by a radio-frequency (RF)electromagnetic field generated by a power transmitter outside the body,so as to provide operating energy to the wireless position sensor; and apassive energy transfer amplifier, which is adapted to be placed inproximity to the wireless position sensor so as to enhance inductivedriving of the power circuit of the wireless position sensor by the RFelectromagnetic field, the wireless position sensor transmitting signalsfor determining six position and orientation coordinates of the positionsensor; and a signal processing unit for receiving signals from theposition sensor and determining six position and orientation coordinatesof the position sensor.
 13. An orthopedic implant, comprising: aprosthetic joint comprising first and second joint elements, which areadapted to be implanted in a body of a subject; first and secondwireless position sensors, which are respectively fixed to the first andsecond joint elements so as to transmit position signals indicative ofan alignment of the first and second joint elements, each of theposition sensors comprising a power circuit, which is adapted to bedriven inductively by a radio-frequency (RF) electromagnetic field so asto provide operating energy to the sensors; a power transmitter, whichis adapted to generate the RF electromagnetic field in a vicinity of thebody; and a passive energy transfer amplifier, which is fixed to atleast one of the first and second joint elements so as to enhanceinductive driving of the power circuit of the wireless position sensorsby the RF electromagnetic field; and a signal processing unit forreceiving the position signals and determining six position andorientation coordinates for the first and second wireless positionsensor.
 14. The implant according to claim 13, wherein the prostheticjoint comprises a hip joint, and wherein the first and second jointelements comprise a femur head element and an acetabulum element, andwherein the passive energy transfer amplifier is fixed to the acetabulumelement.
 15. The implant according to claim 13, wherein the prostheticjoint comprises a knee joint.
 16. Invasive medical apparatus,comprising: a catheter, having a distal end, which is adapted to beinserted into a heart of a subject, the catheter comprising a wirelessposition sensor, fixed adjacent to the distal end of the catheter so asto transmit position signals indicative of a position of the catheterwithin the heart, the position sensor comprising a power circuit, whichis adapted to be driven inductively by a radio-frequency (RF)electromagnetic field so as to provide operating energy to the positionsensor; a power transmitter, which is adapted to generate the RFelectromagnetic field in a vicinity of the body; and a passive energytransfer amplifier, which is adapted to be placed in a vicinity of theheart so as to enhance inductive driving of the power circuit of thewireless position sensors by the RF electromagnetic field; and a signalprocessing unit for receiving the position signals and determining sixposition and orientation coordinates for the wireless position sensor.17. The apparatus according to claim 16, wherein the passive energytransfer amplifier is adapted to be placed on a chest of the subjectadjacent to the heart.
 18. The apparatus according to claim 16, whereinthe wireless position sensor comprises a sensor coil, and wherein theapparatus further comprises one or more field generators, which areadapted to generate energy fields in a vicinity of the heart, whereinthe energy fields cause currents to flow in the sensor coil responsivelyto the position coordinates of the medical device.
 19. The apparatusaccording to claim 16, wherein the catheter further comprises one ormore electrodes for sensing electrical activity within the heart.